Lack of proper nutrition is possible but unlikely. I would be concerned about metabolic diseases; kidney problems, diabetes, liver disease, or protein problems. Various gastrointestinal diseases like inflammatory bowel disease or malabsorptive issues can lead to weight. Often the first sign of cancer is weight loss and loss of muscle mass. Have your cat examined by a veterinarian. Initial tests will include blood work and may include imaging (radiographs or ultrasound).
I have a cat also and Are you feeding it enough and if you are go and bring it to the vet their might be something wrong with it and could be fatal